1. Field of the Invention
The present invention relates to composite nickel particles each having a silica coat formed on a nickel (Ni) core, and more particularly, composite Ni particles each having a silica coat improved in oxidation resistance and heat shrink characteristics and a method of preparing composite Ni particles by using an organic Ni composite.
2. Description of the Related Art
A multilayer ceramic capacitor (MLCC) is fabricated by alternatingly laminating dielectric material layers and internal electrode layers one atop another, bonding the laminated structure of layers together by compression, and densifying the laminated structure by hot firing. In the MLCC, the internal electrodes are fabricated generally by forming metal paste from fine metal powder, printing the metal powder on ceramic dielectric sheets, stacking a plurality of the printed dielectric sheets one atop another, heating and compressing the stack of the printed dielectric sheets, and curing the resultant structure in a reducing atmosphere. The internal electrodes have been made conventionally by noble metals such as platinum (Pt) and palladium (Pd). Recently, however, technologies of using base metals such as Ni have been researched and developed.
In fabrication of an MLCC, firing temperature is different according to the composition of a ceramic dielectric material but typically from 1000° C. to 1400° C. for barium titanate (BaTiO3) based dielectric material. However, Ni metal powder when used for the internal electrode material is subject to rapid heat shrink at a temperature from 400° C. to 500° C. which is much lower than the firing temperature. The Ni metal powder used as the internal electrode material is apt to create defects such as delamination and cracks in the firing due to heat shrink difference between ceramic dielectric material and Ni metal powder.
Accordingly, in order to prevent delamination or cracks in the firing, it is preferred to shift rapid heat shrink starting temperature of the Ni metal powder toward a high temperature range to lower heat shrinkage so that the Ni metal powder can have a heat shrink behavior as similar as possible to that of the ceramic dielectric material.
In addition, in a case where a ceramic dielectric material is fired in contact with a metal, the metal is generally oxidized and a resultant oxide has a diffusion coefficient higher than that of the ceramic dielectric material. Thus, at grain boundaries, diffusion easily takes place from a metal oxide of a higher diffusion coefficient into ceramics of a lower diffusion coefficient. Accordingly, in a case where typical paste of Ni metal powder is used, fine particles of Ni metal are oxidized and resultant Ni oxides are diffused into ceramic dielectric layers. As a result, the internal electrodes are destroyed partially or internally defected and ferrites formed damage dielectric characteristics of a portion of the ceramic dielectric material. Accordingly, in order to fabricate a miniature and slim MLCC having ceramic dielectric layers and internal electrode layers without having to damage dielectric characteristics and electric properties, it is preferred for the Ni powder of the internal electrodes to have excellent oxidation resistance.
To reduce heat shrinkage of the Ni metal powder and shift shrink and oxidation starting temperatures to a higher temperature range, several conventional approaches have been proposed, in which oxygen content of the Ni powder is reduced or an oxide coat was formed on the surface of the Ni powder.
Examples of oxides for coating the Ni powder may include single oxides such as TiO2, SiO2, MgO and Al2O3 and composite oxides such as BaTiO3, SrTiO3, Ba1-xCaxTiO3, BaTi1-xZrxO3. Methods of coating the Ni powder may include a spray pyrolysis method (U.S. Pat. No. 6,007,743), a dry mechanical-chemical mixing method (Japanese Laid-Open Patent Application No. 1999-343501) and the like.
In the spray pyrolysis method, it is possible to fabricate Ni powder containing a composite oxide by spraying a solution containing a thermally decomposable compound and a Ni precursor into droplets and thermally decomposing the droplets. However, in the spray pyrolysis method, oxides are formed not only on the surfaces of Ni particles but also inside the Ni particles. Then, the oxides may reside as impurities after the formation of electrodes. On the other hand, in case of oxide-coated Ni powder prepared by the dry mechanical-chemical mixing method, oxide coats do not strongly adhere to the surfaces of Ni particles and thus may be separated from the Ni particles in manufacturing of paste. This makes it difficult to sufficiently prevent heat shrink of the Ni powder in firing and weak oxidation resistance may permit oxidized Ni powder to diffuse into dielectric layers.
In addition, as disclosed Japanese Laid-Open Patent Application No. 2005-163142, a silicon compound and —OH group on metal forms a silica coat on metal by condensation. Korean Patent Application Publication No. 1999-88656 discloses a method of directly attaching to metal particles by adjusting pH. However, while Japanese Laid-Open Patent Application No. 2005-163142 and Korean Patent Application Publication No. 1999-88656 relate to oxide coating of for example silica on the surface of previously prepared Ni metal particles, the present invention pertains to silica coated on the surface of Ni particles simultaneously with the formation of the Ni particles. That is, in the present invention, metal particles and silica coats are formed in one-step process, which is basically different from the two prior arts.
While Japanese Laid-Open Patent Application No. 2005-163142 discloses silica coating by using a silane coupling agent, coordinate bond by using a silane coupling agent of the invention as a raw material of a silica coat is not disclosed therein. Furthermore, since examples are limited generally to copper (Cu), the thickness of the silica coat is not easily controlled and secondary particles of silica are produced in case of silica coating by using TEOS.
Korean Patent Application Publication No. 1999-88656 pertains to a method of coating oxide on metal surface through a typical aqueous reaction, which is basically different from a method of the present invention in which a heated silane coupling agent forms a silica coat by condensation. In addition, Korean Patent Application Publication No. 1999-88656 does not use the silane coupling agent as a raw material of the coat. In this prior art, an oxide layer of fine crystal can be rarely formed and weak bonding force between the coat and the Ni particles restrict oxidation resistance and shrink characteristics.
Accordingly, there are demands for a method of preparing a composite Ni powder having a silica coat which is free from the above-mentioned technical problems, has excellent oxidation resistance and heat shrink characteristics similar to those of a ceramic dielectric material, thereby preventing defects such as delamination and cracks, and thus can be used an internal electrode material in fabrication of an MLCC.